The present invention is directed to a hydraulic tensioner having a pre-loaded spring, or force limiting spring, contacting the upper end of the piston to reduce the peak operating loads in a timing chain system.
Hydraulic tensioners are typically used as a control device for a chain drive system in an automobile timing system. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises or slippage. Prevention of slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage may alter the camshaft timing by several degrees, possibly causing damage. The tension in the chain may vary greatly due to the wide variation in the temperature and the linear thermal expansion among the various parts of the engine. Camshaft and crankshaft induced torsional vibrations cause chain tension to vary considerably. This tension variation results in chain elongation. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain.
A hydraulic tensioner is used to take up the slack in the chain or belt that connects the camshafts to the crankshaft of the engine timing system. A typical hydraulic tensioner is comprised of a housing having a bore, a fluid chamber defined by the bore, and a hollow piston biased in a protruding direction from the bore by a spring. A check valve is also included in the hydraulic tensioner to permit fluid flow from a source of pressurized fluid into the fluid chamber, while preventing back flow in the reverse direction. The force of the chain against the piston in an inward direction is balanced by the resistance force of the fluid and force of the spring in an outward direction.
When the piston tends to move in the reverse direction, the check valve closes to restrict outflow of fluid from the chamber. Only a small clearance between the piston and the housing wall permits some fluid to escape, thereby allowing the piston to retract. In such a fashion, the tensioner achieves a so-called no-return function.
However, a potential problem with hydraulic tensioners is that the no-return function may present difficulties in accommodating tension spikes or surges in the chain. When a timing device operates at its resonant frequency, the chain load increases significantly. The small clearance between the piston and the housing wall is not sufficient to quickly release the hydraulic fluid in the chamber to accommodate the sudden overload of the chain.
One example of a tensioner that addresses this problem is described in Suzuki, U.S. Pat. No. 4,881,927. Suzuki discloses a hydraulic ball-type check valve tensioner having a piston slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve having a sleeve slidably fitted in an auxiliary chamber in communication with the first chamber, with a spring biasing the sleeve into a depressed position to block a discharge part. Oil in the first chamber flows into the auxiliary chamber to force the sleeve against the biasing spring action to unblock the discharge port. Unfortunately, this relief valve may be slow to open and close due to high mass and subject to variable friction between the sleeve and auxiliary chamber wall. This may vary the pressure at which the relief valve operates.
Another example of a hydraulic tensioner that addresses the problem of overload is described in Mittermeier, U.S. Pat. No. 4,507,103. Mittermeier discloses a hydraulic tensioner having a piston slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve in a bore at the protruding end of the piston. This relief valve is a spring-biased ball type valve with the spring set against a threaded throttle plug capping the bore. Oil in the first chamber forces the ball open, upon reaching a set high pressure, and flows into the bore past the throttle plug threads to the atmosphere. Unfortunately, this relief valve may be slow to release large displacements of oil because of the restricted path past the threads and resultant back-pressure build up against the ball.
Accordingly, it is an object of the present invention to provide a tensioner, which can maintain a substantially constant tensioning force. A pre-loaded spring device eliminates the need for an expensive pressure relief valve. A pre-loaded spring device also eliminates expensive wear face materials used in timing systems and especially in extraordinary strong timing chains used in systems with high resonance loads.